To improve uniform distribution of the inlet air still further, it is known to provide such air distribution boxes with a perforated plate near the outlet to function as an equalizer and to prevent turbulence of the air flow at the outlet. Such perforated equalizer plates are of course not required if the air distribution box is used as an air discharge box through which the air is discharged from the ventilated room into the air duct system.
It is frequently necessary for the quantity of air which is delivered or discharged via an air distribution box to be adjustably varied. In known air discharge boxes the air flowrate was adjusted at the outlet into the room which is to be ventilated by variation of the cross-section of the outlet ports through adjustable flaps or plates. This procedure suffers from the disadvantage of preventing uniform flow of the air through the outlet due to the flowrate adjustment on said outlet, an effect which in turn leads to undesirably noise. The action of the perforated equalizer plates provided in inlet air distribution boxes is therefore partially eliminated by the flowrate adjusting means.